The present invention relates to a nonaqueous electrolyte secondary battery, particularly an improvement of the negative electrode thereof.
Nonaqueous electrolyte secondary batteries having a negative electrode of alkali metal, such as lithium or sodium, has a high electromotive force, and, thus, are expected to provide a higher energy density than that of conventional nickel-cadmium or lead acid storage batteries. Many vigorous studies have been made on the nonaqueous electrolyte secondary batteries, particularly those having negative electrodes of lithium.
However, batteries using negative electrodes of metallic alkali metals may suffer dendrite growth during charging operation. This leads to short-circuiting, thereby decreasing the reliability of the battery.
In order to solve this problem, the availability of a negative electrode comprising an alloy of lithium and aluminum or lead has been investigated. In the batteries which employ the negative electrode of this alloy, lithium is absorbed in the alloy during charging operation and dendrite growth does not occur. Thus, a highly reliable battery can be obtained.
However, since the discharge potential of the negative electrode of this alloy is higher than that of metallic lithium by about 0.5 V, the voltage of the battery including this negative electrode decreases by about 0.5 V accordingly. As a result, the energy density of the battery also decreases.
On the other hand, it has been investigated to use intercalation compounds between lithium and carbon, such as graphite, as an active material for the negative electrode. The negative electrode made of intercalation compound is also free from dendrite growth, because lithium is intercalated between the layers of carbon during charging. The negative electrode has about 0.1 V higher discharge potential, compared with metallic lithium, and the battery shows less decrease in voltage. Such negative electrode is considered more preferable. However, this negative electrode active material has a significant drawback as follows. The capacity of intercalating lithium into the interlayer space of carbon is theoretically limited to the quantity defined by the formula C.sub.6 Li at the maximum (available electric capacity is 372 Ah/kg) in the case of graphite. Carbon, a lower crystalline material than graphite, has been proposed extensively as an active material having a capacity of intercalating lithium exceeding the above-mentioned theoretical value.
The above-mentioned carbon materials have unsatisfactory discharge capacities and the capacity decreases greatly with the advance of charge-discharge cycles.
The primary object of the present invention is to provide a negative electrode which realizes a nonaqueous electrolyte secondary battery with a high electric capacity and an exceptionally long cycle life by absorbing lithium during charging operation, thereby preventing lithium from growing dendrites.
Another object of the present invention is to provide a highly reliable nonaqueous electrolyte secondary battery with an even higher energy density and an exceptional cycle life characteristic free from short-circuiting due to dendrites.